The present invention concerns composite products, particularly paper products, comprising fillers flocculated with starch granules prior to combining the flocculated filler particles with cellulosic material and/or polymerized mineral networks and methods for their manufacture.
Paper products are composite products in which cellulosic fibers, particularly wood fiber, are the primary component. In addition to fiber, many other materials and chemicals are added to form the desired product. Paper products often include mineral additives, referred to as fillers, such as clay, calcium carbonate, talc, kaolin, calcium sulphate and titanium dioxide. Paper primarily comprises a web of cellulosic fibers and minor amounts of mineral and/or organic fillers. Fillers are used, as the name implies, to fill spaces bounded by the cellulosic fibers of the web. Fillers also improve certain paper properties including opacity, brightness and printability. Other additives also can be used to form paper having desirable end-product properties, such as pigments, dyes, starch, sizing agents and strength-enhancing polymers.
Cellulosic products can be made using conventional fillers more economically than products made without such fillers, primarily because of the cost of cellulosic material. Traditionally, minerals like Kaolin clay (hydrous aluminum silicate), chalk, ground limestone or marble, (calcium carbonate), talc (hydrous magnesium silicate), gypsum (calcium sulfate) and diatomaceous earth (silicon dioxide) have been used as fillers. Most other fillers are inorganic materials produced synthetically from minerals (e.g., titanium dioxide, synthetic silica, barium sulfate), or by regeneration after purification (e.g., limestone-to-lime-to-precipitated calcium carbonate).
Fillers used to form paper products using methods developed prior to the present invention reduce the strength properties, such as breaking length in kilometers of the product (tensile strength divided by the basis weight times 102 by TAPPI Method T494), as the percent of filler used to make such products increases. FIG. 1 illustrates this effect and shows that as the filler content is raised above about 20% the strength of the paper decreases considerably. Experience also has shown that other undesirable changes occur as the amount of filler used with conventional paper-making processes increases.
Numerous approaches have been proposed and investigated for raising the amount of fillers that can be used to make paper products. A common approach involves bonding filler particles to the fibrous materials. This approach has met with limited success, primarily because (1) the fibrous materials and the fillers are chemically dissimilar, and (2) the cost of the materials used to make the products using this approach.
Chemicals, chemical compositions, and methods directed to solving problems associated with increasing filler and reducing cellulosic material in paper products are known. For example, retention chemicals and compositions are commercially available. These materials have proved satisfactory in solving the problems associated with flocculating and retaining filler particles in the sheet. These prior inventions, however, do not improve the paper strength and therefore do not solve the problems associated with decreasing paper strength with increasing filler content. One possible explanation for this is that while the retention chemicals and compositions are good at flocculating filler particles, they also flocculate filler particles onto the fibers themselves. Paper strength is considered to arise primarily as a result of fiber-to-fiber bonding between adjacent fibers. This fiber-to-fiber bonding occurs at overlapping fiber surfaces. Filler particles flocculated onto the fibers reduce the surface area of the fibers available for this interfiber bonding, and perhaps intrafiber bonding as well, thereby reducing the strength of the paper product.
I. Silica and Silicates as Fillers
Silica and silicates are common materials, and have been used previously as fillers, retention aids, buffers, chelating agents, and coating components for making paper products. In fact, World Minerals Inc. manufactures a line of products, including calcium silicates, to increase bulk, control stickies, increase printability, etc. Thus, when silica or silicate materials are used to make filler or pigment materials, the particle size of the materials is reduced to the classical range (0.1 to 10 micron) for filler particles even if initially the particles are produced in larger sizes. For example, U.S. Pat. No. 4,790,486 describes a process for preparing paper that involves making hydrous silicic acid fillers by wet pulverizing a slurry to 1-30 microns from large particles. U.S. Pat. No. 5,030,284 discloses a spray drying technique of gelled alumina-silica-sulfate compositions to generate 1-10 micron particles.
Silica and silicate have been used as retention/drainage aids for the production of paper. For example, U.S. Pat. No. 5,127,994 describes using silica-based colloids. The colloidal particles have a particle size of 4 to 7 nanometers. U.S. Pat. No. 4,643,801 describes an improved binder system containing three ingredients, a cationic starch, anionic polymer, and dispersed silica having a particle size of 1-50 nanometers (0.001 to 0.05 microns). This system requires the three components and utilizes small dispersed silica. This seems to be a variation of U.S. Pat. Nos. 4,385,961 and 4,388,150, which have a binder system of colloidal silicic acid and cationic starch.
Other variations of these retention/drainage aids include the formation of microgels, which are three-dimensional chain networks formed of particles having a diameter of 1-2 nanometers. These small colloidal particles are stabilized to prevent further growth or gellation. See, for example, U.S. Pat. Nos. 4,954,220, 5,279,807 and 5,312,595. Neutralization of alkali silicate solutions forms polysilicic acid (from polymeric anions), which polymerizes to form microgels comprising three dimensional aggregates of very small particles of polysilicic acid. The formation of polysilicate microgels is initiated by the addition of an acidic material (aluminum sulfate, sodium stannate, sodium orthoborate decahydrate, acid ion exchange resins, sodium aluminate, etc.). The xe2x80x9cinitiatorxe2x80x9d starts the gelation (polymerization) process, which is stopped before total gelation of the solution. This process is done independent of the papermaking process. It is then added to the system as any other retention/drainage additive. See, FIG. 2, which shows the polymerization behavior of silica. Polysilicate microgel has been found to constitute a good retention and drainage aid when combined with a water-soluble cationic polymer.
Kaliski""s U.S. Pat. No. 5,240,561 discloses the use of microgels formed similarly from alkali silicate solutions and a second aqueous solution of sodium aluminate or sodium zincate. Kaliski""s patent concerns a process for making paper and teaches the formation of microgels in situ (in the furnish). But his microgels, described as transient, chemically reactive, subcolloidal sodium-silico-aluminate or similar microgels, must be crosslinked with bivalent and/or multivalent inorganic salts (like calcium chloride) prior to the formation of colloidal particles (Tyndall effect). This crosslinking immediately stops the gelation process (polymerization) and precipitates the calcium crosslinked microgel. Kaliski demonstrates the use of this procedure for papermaking as a new flocculation mechanism where precipitation of the microgel coagulates and flocculates all particulates present in the papermaking furnish. Thus, Kaliski""s patent is a variation of the previous patents for retention and drainage aid but is done in situ. Kaliski""s patent requires a specific order of addition (1st-sodium silicate, 2nd-sodium aluminate, 3rd-calcium chloride) with a termination of the polymerization at the subcolloidal stage of growth. Kaliski""s U.S. Pat. Nos. 5,116,418 and 5,279,663 discusses using this flocculation technology to generate pigments. Kaliski""s U.S. Pat. No. 5,378,399 discusses the formation of functional complex microgels.
The patents discussed above concerning colloidal particle and/or microgels, demonstrate no evidence of formation of large three-dimensional mineral networks about the fiber. The initial particles or microgels are very small (usually 1-50 nanometers and less than 100 nanometers or 0.1 micron) and larger particles are actually detrimental to the properties of the silica/silicate. The concentration of the silica/silicate and the subsequent dilution of the materials in the papermaking process would preclude the formation of large mineral networks.
Several publications teach that the formation of large networks of mineral products are not desired for the formation of paper products. For example, U.S. Pat. No. 3,720,531 states that xe2x80x9cas is well known, the product obtained by simply neutralizing alkali silicates is useful as adsorbents and catalysts, but this cannot be used as pigment.xe2x80x9d Emphasis added. A similar statement is made in an article titled xe2x80x9cPreparation of an Artificial Silicate Filler,xe2x80x9d by S. N. Ivanov and V.V. Kuznetski. xe2x80x9cIt is well known that during processing of water glass (sodium silicate) with acid or solutions of transition metal salts under ordinary conditions gels are formed, which as a result of their structure and properties, cannot serve as fillers.xe2x80x9d Emphasis added.
Japanese patent Kokai 239795/93 discusses the use of calcium silicate to make an ignition resistant paper, and U.S. Pat. No. 5,372,678 discloses making a calcium hydrosilicate-bound fiberboard by a molding/pressing operation without using conventional fourdrinier.
II. Preflocculating Filler Particles
Methods also have been developed to improve paper strength while increasing filler loading. One such technique involves preflocculating filler particles before the filler particles are added to the cellulosic fiber material. The preflocculated filler agglomerates are better retained by the web created by the fibrous material and are not flocculated onto the fiber. This leaves more fiber surface area available for interfiber bonding, which also increases the strength of the paper product. The methods used to preflocculate filler particles generally are based on the same chemistry as retention materials that are applied in the wet end of the paper making process. These include charge neutralization and using retention chemicals to bridge between filler particles. Similarly charged filler particles repel each other. Charge neutralization involves neutralizing the charges of the particles to alleviate this repulsive force, thereby allowing the particles to flocculate. The chemicals used for these processes include cationic starch, high-molecular-weight cationic or nonionic polymers, or combinations of cationic starch and polymers. Because cationic starch contains positive charges which can flocculate negatively charged pigment particles, it is commonly used in the wet end of the paper making process to flocculate fiber fines.
While preflocculating filler particles has advantages, it also has disadvantages. One disadvantage is that it has proved difficult to control the size of the agglomerates produced by the flocculation process. This can result in the formation of large agglomerates, which appear as visible imperfections in the final paper product, or the agglomerates are too small, which requires the use of retention aids to retain the agglomerates in the sheet.
As stated above, starches have been used to preflocculate filler particles, and some of these inventions have been patented. These inventions involve using starch polymers to flocculate the filler particles, with agglomerates of desired sizes being produced by, for example, subjecting the agglomerates to high shear forces with complicated process designs. One example of a method for flocculating filler particles using starch polymers is Richard Davidson""s U.S. Pat. No. 4,151,187.
Although starches have been used to form paper products prior to the present invention, conventional wisdom in the art of paper making is to use cationic starches in the wet end of the paper making process to enhance retention of filler fines and pigment particles. Furthermore, most starches are chemically modified from their natural form prior to being used as flocculating agents. For example, corn starches often are chemically treated, such as by ethylation, to change the naturally occurring chemical structure. The modified starch is then cooked, which breaks the starch up so that it dissolves in water. It can then form a film on the fiber. Starch also is cationized, i.e., chemically treated to provide the material with positive charge so that it is electrostatically attracted to the fiber portion of the paper, which is negatively charged. Starch modification and cationization add processing steps and expense to the overall paper-making process.
The present invention provides a new approach that in one aspect addresses the filler concentration-strength problem illustrated by FIG. 1, and further demonstrates a fundamentally different approach to the composition, structure and formation of cellulosic products. Past attempts to overcome strength reduction limits of paper products made with fillers were limited by the basic principle that strength is derived primarily from the bonding together of fibers. Prior to the present invention, people of ordinary skill in the art of paper making generally did not consider bonding filler particles together during the formation of paper products beneficial because this reduces the number of individual filler particles, which decreases both product opacity and bulk.
The present invention departs from the concept of relying solely on the bonding together of fibers in cellulosic products to provide adequate strength. The products of the present invention are not just bonded fibrous mats with fillers, but rather comprise composite structures having filler particles bonded to each other (and perhaps also to the cellulosic material) to form a mineral network by polymerization reactions about the cellulose. The result is a new composite product having properties different from conventional paper products. One reason for this may be because properties of the composite product, such as strength, In stiffness, opacity, printability, etc., are derived from the bonding together of fiber, the filler network polymerized generally at least partially in the presence of the fiber, and in some embodiments about the fiber, and perhaps bonding between the network and the fiber.
Certain embodiments of the composite products of the present invention differ from conventional paper products in that the method used to produce such products, referred to herein as Ca-FLOCC purposefully results in the formation of a mineral network, such as a silica/silicate network polymerized around, throughout and/or surrounding the fiber material, which is referred to herein as forming a mineral network xe2x80x9caboutxe2x80x9d the cellulosic material. This is in contrast to conventional processes which deposit isolated particles of filler material onto or bond such filler particles to the cellulosic material. In other embodiments, the formation of the mineral network is initiated outside the presence of the fiber, and then continues after being combined with the fiber, referred to herein as Mg-FLOCC and Si- FLOCC.
One difference between the composite products of the present invention and conventional paper products is illustrated in FIGS. 3-6. SEM images of paper products made according to the present invention, particularly by the Ca-FLOCC method, are clearly distinguishable from paper products made using prior methods, and further show that the Ca-FLOCC method, and to a lesser extent Mg-FLOCC and Si-FLOCC methods produce a mineral network about the fibers. SEM images of ash (i.e., primarily the combustion product of materials added to form the product other than cellulose) of cellulosic products made according to the Ca-FLOCC method still resemble polymerized networks, as opposed to the more friable ash produced from products made having individual filler particles deposited onto or bonded to the cellulosic material as with conventional paper products. See, FIGS. 7-10.
The products of the present invention are usually much stronger than conventional paper products with the same ash content. This is achieved while significantly increasing filler amounts relative to amounts thereof commonly found in paper products made having isolated individual particles of filler product distributed on or bonded to the cellulosic material.
The present invention also concerns a method for making the composite products. The method comprises polymerizing a mineral network at least partially in the presence of the cellulosic fiber, or about a cellulosic fraction. The simplest and easiest approach to forming products by the method Si-FLOCC involves acidifying, such as by adding carbon dioxide to a Group I metal silicate material to initiate polymerization, and then adding this material to cellulose. Another approach to polymerizing the mineral network comprises providing a mixture comprising cellulosic material, a Group I metal silicate, particularly sodium silicates, and a Group II metal base or salt, particularly calcium and magnesium salts, examples of which are calcium and magnesium oxide and calcium and magnesium hydroxide. This mixture is then acidified, and perhaps simultaneously carbonated, to produce a product comprising precipitated carbonate filler material and a polymerized mineral network about the cellulosic material. This product can be formed into common cellulosic products, such as paper products. Carbonating the mixture has been accomplished by combining carbon dioxide with the mixture, but other conventional methods of carbonating aqueous mixtures also can be used.
The mixture typically comprises: (1) from about 40% to about 60%, by weight, (all percents stated herein are by weight relative to the oven dried solids of the stock unless noted otherwise) cellulosic material, particularly delignified cellulose, (2) from about 0.5% to about 20% Group I metal silicates, particularly sodium silicate, and (3) from about 0.5 to about 45% of a Group II metal salt, such as calcium oxide, with the Group II metal salt content being generally equal to, and more typically greater than, the silicate content. The mixtures are then formed into paper products. The mixture also may include other materials commonly used in the formation of paper products, such as but not limited to, materials selected from the group consisting of fillers, such as precipitated calcium carbonate, starch, retention aids, brighteners, biocides, sizing agents, pigments (TiO2, clay, talc, etc.), and mixtures thereof.
The present method provides several advantages over prior methods. For example, much higher filler contents can be used in the formation of products with concomitant cost savings. Moreover, precipitated filler material, such as precipitated calcium carbonate, also can be produced in situ during formation of the product, which ensures that good quality filler is used and is evenly distributed throughout the product, and perhaps incorporated into the mineral network.
The present invention also provides a method for preflocculating filler particles useful for making paper products. The method comprises providing a starch, processing the starch to form starch granules, and and forming filler particle-starch floccules by mixing filler particles with the starch granules. The starch generally is selected from the group consisting of leguminous starches, potato starch, corn starch, and mixtures thereof, preferably pea starch. The step of processing the starch typically comprises providing an aqueous starch dispersion comprising from about 1 to about 10 weight percent starch, and heating the dispersion to a temperature of from about 60xc2x0 to about 120xc2x0 C., preferably from about 70 to about 90xc2x0 C. The average size of the starch granules is typically is from 15 xcexcm to about 150 xcexcm, and preferably from about 40 xcexcm to about 70 xcexcm
The present invention also provides a method for making a paper product comprising: preflocculating filler particles useful for making paper products by processing a starch to form starch granules and forming filler particle-starch floccules by mixing filler particles with the starch granules; providing a cellulosic fraction; and forming a mineral network about the cellulosic fraction simultaneously with or subsequent to combining the filler particle-starch floccules to the cellulosic fraction.